Not Applicable.
Not Applicable.
1. Scope of Invention
The invention relates to an improved time digitizer. More specifically, a time digitizer with improved time resolution and pulse-pair resolving time is described.
2. Description of the Related Art
Precision measurements of time intervals are important in many areas of science. One example is a time-of-flight mass spectrometer (TOF-MS). A TOF-MS is used to measure the mass spectrum of a chosen sample. The sample is injected into the TOF-MS system by one of a variety of techniques. Subsequently, the sample is ionized and accelerated toward an ion detector by a fixed amplitude voltage pulse. The TOF-MS system measures the mass of the molecules or molecular fragments by measuring the time required for the accelerated ions to travel the fixed distance of the TOF-MS chamber. Ions with the lowest mass and highest electrical charge arrive first at the ion detector. Heavier ions arrive later. A mass record is measured by recording the arrival time of each of the ions accelerated by a given voltage pulse. By repeating the process of injection, acceleration, and time measurement many times and combining the mass records obtained, a statistically significant graph of the total number of detected ions versus flight time is accumulated. This graph, called the mass spectrum, gives the relative number of molecules of different mass in the sample and is a valuable analytical tool.
Measurement of an accurate mass spectrum with the TOF-MS is technically difficult. The total time measurement interval for the heaviest ions may be several hundred microseconds while the time resolution required to accurately identify a given mass species might be a few hundred picoseconds. One method of measuring the flight time to a precision of a hundred picoseconds is to count the cycles of an electronic clock running at 10 gigahertz. Each cycle represents one hundred picoseconds of elapsed time so the number of cycles between the ion acceleration time and the final arrival time is the time of flight in 100-picosecond units. As each ion arrives, the counter total is written to a data list in memory. The list of flight times for each injection cycle is a mass record and the sum of many mass records produces the mass spectrum. A serious problem with a method of this type is the time required to store the arrival time. Reading the counter and storing the resulting value requires several nanoseconds, even using the fastest known memories. The result of such a measurement technique is time precision of one hundred picoseconds but an inability to measure two events occurring a few nanoseconds apart. In a TOF-MS, this measurement problem results in a serious error. An ion species having a mass just slightly higher than a species common in the sample will not be correctly measured. This error occurs because the heavier ion arrives at the detector during the dead time that is required to store the arrival time of the lighter ion.
In many areas of time measurement, the need to resolve closely spaced events does not imply a high average data rate. In the TOF-MS, for example, events separated by about one nanosecond must be resolved in order to prevent distortion of a mass peak by a nearby lighter ion species. This implies a burst data rate of one gigahertz. The actual average data rate is usually less than one megahertz.
It is an object of the invention to provide a design for a time digitizer having high precision and the ability to resolve closely spaced events without incurring the cost and complexity of a design capable of extremely high data rates.
A method and apparatus for precision time interval measurement in a time-of-fight mass spectrometer (TOF-MS) is provided. An asynchronous serial stream of data, consisting of a start pulse followed by an arbitrary number of stop pulses, repeated an arbitrary number of times, is converted into a digital stream of data synchronized to a precision master clock. Conversion of the asynchronous, analog data to synchronous digital data simplifies the measurement task by allowing the use of powerful, low-cost digital logic in the measurement
The synchronous digital data stream is separated into a sequence of digital words of fixed length, such that the bit pattern in the digital words is the same as the bit pattern in the serial digital data stream. Converting the data from a serial stream to a parallel stream reduces the effective data rate.
In principle, two separate channels could be used to analyze the start and stop pulses separately. The time difference between a start pulse and a subsequent stop pulses would be obtained in later processing. In practice, it is more cost-effective to combine the start and stop pulses into a single stream of data, distinguished by their bit pattern. A time interval with no pulses present is represented by a sequence of digital zeroes, one zero for each cycle of the master clock. A start pulse is represented by a short sequence of digital ones. A differing sequence of digital ones represents a stop pulse, thereby distinguishing the stop pulse from a start pulse.
The sequence of digital words is analyzed by a digital signal processing system. When the bit pattern representing a start pulse is recognized, the word number and bit position of the leading one is recorded in the output data stream. When the bit pattern representing a stop pulse is recognized in the same word as the start pulse or some word occurring later in the data, the word number and bit position of the leading one is recorded in the output data stream.
The data output of the signal processing system consists of a sequence of digital words containing a type marker, denoting whether the pulse was a start or stop pulse, followed by an input data word number and a bit position in that word. The width of the output data word is chosen so that the maximum desired time interval can be represented. Data words containing all zeroes are quickly recognized and counted. The described data processing results in an output data stream more compressed than the input data stream.
Final analysis to produce the mass spectrum is very simple. If the input data words are numbered starting from zero and the bit position is numbered from the left starting from zero, then the clock time for a given pulse is calculated by the equation:
Time=[(Word Number)*(Word Length)+(Bit Position)]*(Clock Cycle Time).
The mass record is calculated by subtracting the start time from each stop time value until a new start time is reached. The mass records are combined to form the mass spectrum.